1. Field of the Invention
The invention relates to an apparatus for rotating the polarization of an entering light beam.
2. Background Art
In order to create faster and more sophisticated circuitry, the semiconductor industry continually strives to reduce the size of the circuit elements. The circuits are produced primarily by photolithography. In this process, the circuits are printed onto a semiconductor substrate by exposing a coating of radiation sensitive material to light. The radiation sensitive material is often referred to as a “photoresist” or just resist. Passing the light through a mask, which may consist of a pattern of chrome or other opaque material formed on a transparent substrate, generates the desired circuit pattern. The mask may also be formed by a pattern of higher and lower regions etched into the surface of a transparent substrate, or some combination of the two techniques. Subsequent thermal or chemical processing removes only the exposed or only the unexposed regions of the resist (depending on the material) leaving regions of the substrate bare for further processing which in turn produces the electronic circuit.
In lithography, projection exposure systems with a high numerical aperture are necessary in order to achieve the highest resolutions. Typically, light is coupled into the resist layer at relatively large angles. When this light is coupled in, the following occur: light losses because of reflection at the outer resist boundary layer and deterioration of the resolution because of lateral migration caused by reflections at the two boundary layers of the resist to the wafer and to the air (formation of standing waves).
In lithography, the polarization of the light can have a substantial impact on the imaging. For example, polarization at the reticle affects the lithographic performance of the lens in several ways. First, the interaction of the illumination with features of the reticle, for example, dense lines of chrome, varies with polarization. Accordingly, the transmission and scattering of the mask depend on the polarization of the light and the features of the mask. Second, reflections at the surfaces of the lenses and mirrors are polarization dependent so that the apodization and, to a lesser degree, the wave front of the projection optics (“P.O.”) depend on polarization. Also, the reflection from the surface of the resist depends on polarization, and this too is effectively a polarization dependent apodization. Finally, the rays diffracted from the reticle that are brought back together at the wafer must interfere to produce an image. However, only parallel components of the electric field can interfere, so the polarization state of each ray at the wafer affects the coherent imaging. Even with a perfect lens, the three dimensional geometry of the rays arriving at the wafer can reduce the contrast.
One reason for considering a polarized illuminator, therefore, is to improve the image formed at the wafer by improving the interference of the diffracted rays at the wafer. This is particularly useful in high numerical aperture systems. For example, if dipole illumination is incident on a binary mask of dense lines each small region in the illuminator pupil (i.e. each pole of a low sigma dipole) is incoherent with other regions in the pupil and makes its own image at the wafer, so one can consider a single pole of the dipole illumination. The light diffracts from the reticle, and the dense lines produce tight diffraction orders. For small features, only two diffraction orders are accepted into the P.O. At the wafer, these diffraction orders recombine to form an image of the mask. For example, if the incident light is polarized so that these diffraction orders are P polarized at the wafer, the electric field of the two diffraction orders are not parallel, and they do not interfere well. On the other hand, if the diffraction orders are S polarized at the wafer, the electric field is parallel and the contrast is enhanced.
Accordingly, in lithography it is desirable to enhance contrast and improve imaging (or increase throughput and dose) at the wafer by controlling polarization at the relevant surfaces.
One approach is to provide a polarizing filter. Such conventional polarizing filters, however, act to polarize light uniformly across an exposure beam. No custom pattern of polarization is provided so that portions of an exposure beam are polarized differently. Further, such polarizers act to subtract light components which reduces transmissivity by at least 50%.
Another approach has been to provide a single layer of mosaic tiles that non-subtractively rotate light. Individual tiles are wave plate facets arranged to provide a limited radial polarization pattern. See, U.S. Pat. No. 6,191,880. Mosaic tiles made of birefringent material (such as naturally birefringent crystals) are especially vulnerable to differential rates of thermal expansion. This can prevent the tiles from being supported through optical contact. Non-contacted surfaces introduce an uncontrolled gap that can result in significant apodization (that is, intensity variation across the exposure beam). Moreover, tiles made of birefringent materials typically have very poor angular acceptance.